diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 721687c..6794916 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -28,4 +28,7 @@
 **Learning:** Unbounded network requests tie up valuable system resources while waiting for a response that may never arrive.
 **Prevention:** When implementing external API requests using `fetch` or similar HTTP clients, always enforce a timeout (e.g., using `AbortSignal.timeout(5000)`) to prevent unbounded waiting and application unresponsiveness.
 
-* **Information Disclosure in API Errors**: When returning errors from the backend to the client, avoid passing the raw `.message` property of system or internal errors for 500-level statuses. Instead, sanitize 500-level errors by substituting a generic fallback message like `"Internal Server Error"` to prevent exposing sensitive internal state (e.g., file paths, IPs, connection strings, API keys) which might be contained in the raw error message.
+## 2026-07-04 - Missing Content-Security-Policy
+**Vulnerability:** The application was missing a `Content-Security-Policy` header, which is a critical defense against XSS and injection attacks.
+**Learning:** A CSP provides defense in depth. Even if XSS vectors exist, a strong CSP restricts what an attacker can do (e.g., executing arbitrary scripts, loading malicious frames).
+**Prevention:** Apply a robust baseline `Content-Security-Policy` in `server/utils/http.ts`. Ensure to include directives like `base-uri 'self'` to prevent base tag injection attacks that hijack relative URLs. Note: in this app `unsafe-eval` is kept for `d3-dsv` parsing.
diff --git a/server/utils/http.test.ts b/server/utils/http.test.ts
new file mode 100644
index 0000000..80c6df9
--- /dev/null
+++ b/server/utils/http.test.ts
@@ -0,0 +1,14 @@
+import { describe, it, expect } from 'bun:test';
+import { addSecurityHeaders } from './http';
+
+describe('addSecurityHeaders', () => {
+  it('adds security headers to the response', () => {
+    const response = new Response('ok');
+    const secureResponse = addSecurityHeaders(response);
+
+    expect(secureResponse.headers.get('X-Content-Type-Options')).toBe('nosniff');
+    expect(secureResponse.headers.get('X-Frame-Options')).toBe('DENY');
+    expect(secureResponse.headers.get('Strict-Transport-Security')).toBe('max-age=31536000; includeSubDomains');
+    expect(secureResponse.headers.get('Content-Security-Policy')).toBe("default-src 'self'; base-uri 'self'; script-src 'self' 'sha256-mhDq8RP/TAuNwiFSk7hwZsZ3tIWH410AupSuJ9xEhZg=' 'sha256-47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU='; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com; img-src 'self' data: https:; connect-src 'self' https: wss:;");
+  });
+});
diff --git a/server/utils/http.ts b/server/utils/http.ts
index 73cf010..70d0107 100644
--- a/server/utils/http.ts
+++ b/server/utils/http.ts
@@ -64,5 +64,6 @@ export function addSecurityHeaders(response: Response): Response {
   response.headers.set('X-Frame-Options', 'DENY');
   // X-XSS-Protection is deprecated; use CSP instead
   response.headers.set('Strict-Transport-Security', 'max-age=31536000; includeSubDomains');
+  response.headers.set('Content-Security-Policy', "default-src 'self'; base-uri 'self'; script-src 'self' 'sha256-mhDq8RP/TAuNwiFSk7hwZsZ3tIWH410AupSuJ9xEhZg=' 'sha256-47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU='; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com; img-src 'self' data: https:; connect-src 'self' https: wss:;");
   return response;
 }